Don't reallocate send buffer for every batch of transmits

quinn/src/connection.rs

@@ -797,6 +797,7 @@ impl ConnectionRef {
                 ref_count: 0,
                 socket,
                 runtime,
+                send_buffer: BytesMut::new(),
                 buffered_transmit: None,
             }),
             shared: Shared::default(),
@@ -876,6 +877,7 @@ pub(crate) struct State {
     ref_count: usize,
     socket: Box<dyn AsyncUdpSocket>,
     runtime: Arc<dyn Runtime>,
+    send_buffer: BytesMut,
     /// We buffer a transmit when the underlying `poll_send` can't complete
     buffered_transmit: Option<udp::Transmit>,
 }
@@ -886,14 +888,13 @@ impl State {
         let mut transmits = 0;
 
         let max_datagrams = self.socket.max_transmit_segments();
-        let capacity = self.inner.current_mtu();
-        let mut buffer = BytesMut::with_capacity(capacity as usize);
+        self.send_buffer.reserve(self.inner.current_mtu() as usize);
 
         while let Some(t) = self.buffered_transmit.take().or_else(|| {
             self.inner
-                .poll_transmit(now, max_datagrams, &mut buffer)
+                .poll_transmit(now, max_datagrams, &mut self.send_buffer)
                 .map(|t| {
-                    let buffer = buffer.split_to(t.size).freeze();
+                    let buffer = self.send_buffer.split_to(t.size).freeze();
                     udp_transmit(t, buffer)
                 })
         }) {